Generally, in a fuel cell system provided with a solid oxide fuel cell (SOFC), a hydrocarbon-based fuel and water are needed for reforming reaction. Therefore, in a conventional fuel cell system, gas (hereinafter, referred to as “anode off-gas”) containing moisture which is discharged from an anode of the fuel cell is recirculated to a reformer using a circulation pump.
However, when the anode off-gas is recirculated, a circulation pump provided with a performance withstanding a high temperature comparable with the temperature of the anode off-gas becomes necessary. Furthermore, there is a problem that a volumetric flow rate becomes large due to high temperature of the anode off-gas, and dimensions of the circulation pump become large. In Japanese Patent Laid-Open No. 2006-302660, it is disclosed that for coping with this problem, since heat exchange is carried out between an oxidant (for example, air) introduced into a cathode of a fuel cell and the anode off-gas to be circulated, the anode off-gas supplied to the circulation pump can be cooled, heat resistance performance of the circulation pump can be lowered, and the volumetric flow rate of the anode off-gas can be decreased.